This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-011961, filed Jan. 21, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a gravimeter for measuring gravitational acceleration, particularly to an absolute gravimeter for measuring the gravity at a specific place.
2. Description of the Related Art
A gravimeter has been used to estimate the rise or drop of the ground, changes of materials in the earth, and also the nature of soil from a gravity distribution, from changes of gravitational acceleration, for example, in the field of geophysics.
Gravimeters are basically classified into absolute gravimeters and relative gravimeters. An absolute gravimeter measures the absolute value of gravitational acceleration (9.79 . . . m/s2) with a precision of eight to nine figures.
On the other hand, a relative gravimeter measures the gravity difference between two measurement points, or changes in gravity over time at one measurement point, and a LaCoste gravimeter is well known. A relative gravity meter is generally small-sized, and is superior in mobility, but cannot be used in observing a long-period fluctuation because of the drift of a spring. Therefore, a relative gravitational difference with respect to a certain gravitational reference point is measured, and then the absolute value is estimated. However, even in this case, schemes such as reciprocating measurement in a short time are required for correcting the drift.
At present, only the FG5 absolute gravimeter, manufactured by Micro-g Solutions Co., U.S.A., is available on the market. In the FG5 absolute gravimeter, a drop mirror is raised by a mechanical elevator, and then simply dropped. The relation between the current position of the drop mirror and an elapsed time is obtained, to obtain the gravitational acceleration value. An interferometer disposed between a reference mirror supported by the spring and the drop mirror is used to count interference fringes, and the position of the drop mirror is measured.
Since the FG5 absolute gravimeter counts the interference fringes to specify the position of the drop mirror, position data of the drop mirror have a poor position resolution of a half wavelength. The drop distance, therefore, is set long to about 20 cm in order to obtain a sufficient number of data. Therefore, the apparatus is large. Moreover, it is necessary to correct contribution of gravity gradient depending on the vertical position with respect to an obtained gravity value. Furthermore, since an optical path difference between a light beam passed via the reference mirror and a light beam passed via the drop mirror is large, it is also necessary to correct the frequency fluctuation error of the laser beam source.
It requires about ten seconds at minimum to raise the drop mirror by the mechanical elevator, and therefore the apparatus is unsuitable for continuous measurement. Moreover, the mechanical operation in vacuum has a low reliability.
Additionally, it is necessary to correct several system errors regarding vacuum pressure, speed of light, and the like. That is, since the drop mirror moves in only one direction, it is necessary to correct the resistance of molecules of air, and the like remaining in a chamber, or the influence of the relative speed of light (influence exerted because the speed of the light has a finite value).
The reference mirror is supported by the spring and is hardly influenced by high-frequency ground vibration, but the influence of low-frequency ground vibration cannot be avoided, and the resolution is actually determined by the vibration inherent in the spring excited by the low-frequency ground vibration.
Absolute gravity measurement is important for observing or forecasting earthquakes or volcanic activity. However, a conventional absolute gravimeter has a very large size, and lacks the mobility required for outdoor observation. Moreover, since apparatuses are very expensive, the apparatuses have not been installed in a large number of locations for emergencies such as a volcanic eruption under existing circumstances.
The present invention has been developed in consideration of the above-described circumstances, and a main object thereof is to provide a small-sized and inexpensive absolute gravimeter.
According to an aspect of the present invention, there is provided a micro-distance toss-up type absolute gravimeter comprising: a vacuum container mounted on the ground; a reference mirror supported in the vacuum container; a drop mirror to be dropped within the vacuum container; a toss-up device which tosses up the drop mirror by a constant distance; an atomic clock which can define time with a high precision; an interferometer which measures the position of the drop mirror in time series, and can acquire an interference waveform; and an operation unit which calculates gravitational acceleration based on the interference waveform obtained by the interferometer and the time defined by the atomic clock.
According to another aspect of the present invention, there is provided a micro-distance toss-up type absolute gravimeter comprising: a vacuum container mounted on the ground; a reference mirror supported in the vacuum container; a drop mirror to be dropped within the vacuum container; a toss-up device which tosses up the drop mirror by a constant distance; an atomic clock which can define time with a high precision; an interferometer which measures the position of the drop mirror in time series; a support mechanism to support the reference mirror in such a manner that the reference mirror can be displaced in a direction vertical to the ground; a driving mechanism which vibrates the reference mirror in the same phase as that of the ground; and an operation unit which calculates temporary gravitational acceleration based on information from the interferometer and the atomic clock, subtracts an acceleration of vibration of the ground obtained based on a driving signal of the driving mechanism from the calculated temporary gravitational acceleration, and calculates the gravitational acceleration.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.